1. Field of the Invention
The invention relates to an actuator that drives a state changeover element, which changes state by being pushed/pulled in its axial direction.
2. Description of the Related Art
In general, an automatic transmission for a vehicle is so constructed as to establish a shift range selected from a parking range (P), a reverse range (R), a neutral range (N), a drive range (D), and the like through, for example, the operation of a shift lever installed near the driver seat of the vehicle (e.g., see Japanese Patent Application Publication No. 5-118436 (JP-A-5-118436) and Japanese Patent Application Publication No. 2005-198450 (JP-A-2005-198450)).
In general, such a range changeover device is constructed to change the state of a manual valve as one component of a hydraulic control unit for shift range changeover when the shift lever is operated.
As for a drive system for changing the state of this manual valve, as described in Japanese Patent Application Publication No. 5-118436 (JP-A-5-118436) and Japanese Patent Application Publication No. 2005-198450 (JP-A-2005-198450), a range position selected by the shift lever operated by the driver is detected through the use of a sensor or the like, a tilt member called a detent plate is tilted by a prescribed angle with the aid of an actuator such as an electric motor or the like, and a spool of the manual valve is thereby axially displaced by this detent plate.
When the parking range P is selected by the shift lever, the parking rod of a parking device is axially displaced as the detent plate is tilted. Thus, the output shaft of the automatic transmission is locked to prevent rotation of the output shaft.
The parking rod is axially actuated by tilting the detent plate in response to the operation of putting the shift lever into the parking range P or removing the shift lever from the parking range P.
As for the coupling between a tilt spindle of the detent plate and an output shaft of the actuator, as described in Japanese Patent Application Publication No. 5-118436 (JP-A-5-118436) in particular, a coupling end of the tilt spindle, which is rotatably supported at an appropriate location in an automatic transmission case, and a transverse hole for coupling, which is provided at one end of the output shaft of the actuator, both assume, for example, a generally oval shape. The tilt spindle of the detent plate and the output shaft of the actuator are simply fitted to each other to rotate together. In addition, the tilt spindle of the detent plate and the output shaft of the actuator may also be coupled to each other through a spline fitting.
In the foregoing conventional examples, if a failure in a control system of the actuator results in an overrun, namely, rotation of the output shaft of the actuator and the tilt spindle of the detent plate coupled thereto beyond a target rotational angle, the detent plate tilts more than necessary.
The mechanism described in JP-A-2005-198450 takes into account the fact that the overrun phenomenon, such as an excess over the target rotational angle, occurs due to an inertial force resulting from high-speed rotation of an electric motor, which may be used as the actuator, and describes a technique for preventing the occurrence of overrun by checking the load of the actuator when the actuator is driven and controlling the rotational speed of the actuator.
In JP-A-2005-198450, unlike the invention, there is no technical concept of making the tilt member physically unrotatable even in a situation where the actuator is driven more than necessary as a result of the uncontrollability of the rotational angle of the actuator.
However, if the art described in JP-A-2005-198450 is applied, it seems possible to take measures by incorporating a failsafe program into the control system so as to forcibly stop the driving of the actuator when the control system itself of the actuator recognizes a situation where the actuator is uncontrollable.
In that case, however, when an error arises in the failsafe program of the control system, the overrun of the actuator cannot be prevented. There is a room for improvement in this respect.